Conventionally, incombustible HFC refrigerant such as R410A has been used in a heat pump apparatus. As R410A has zero ozone depletion potential (hereinafter referred to as “ODP”), which is different from conventional HCFC refrigerant such as R22, there is no risk of disrupting the ozone layer. However, R410A has a characteristic of high global warming potential (hereinafter referred to as “GWP”). As such, as part of prevention of global warming, a shift from HFC refrigerant having high GWP, such as R410A, to refrigerant having low GWP is being considered.
Refrigerant candidates of such low GWP include HC refrigerant such as R290 (C3H8; propane) and R1270 (C3H6; propylene) that are natural refrigerants. However, R290 and R1270 have high-level combustibility (high combustibility), which is different from incombustible R410A. As such, it is necessary to pay attention to leakage of refrigerant when using R290 or R1270 as refrigerant.
Further, refrigerant candidates of low GWP also include HFC refrigerant having no carbon double bonds in its composition such as R32 (CH2F2; difluoromethane) having lower GWP than that of R410A.
Further, as a similar refrigerant candidate, there is halogenated hydrocarbon that is a kind of HFC refrigerant, similar to R32, and has carbon double bonds in its composition. Such halogenated hydrocarbon includes HFO-1234yf (CF3CF═CH2; tetrafluoropropene) and HFO-1234ze (CF3—CH═CHF), for example. It should be noted that HFC refrigerant having carbon double bonds in its composition is likely to be expressed as “HFO” by using “O” standing for olefin (unsaturated hydrocarbon having carbon double bonds is called olefin), to be distinguished from HFC refrigerant having no carbon double bonds in its composition such as R32.
Such low-GWP HFC refrigerant (including HFO refrigerant) has slight-level combustibility (slight combustibility) that is different from incombustible R410A, although it is not highly combustible like HC refrigerant such as R290 that is natural refrigerant. As such, similarly to the case of R290, it is necessary to pay attention to leakage of refrigerant. Hereinafter, refrigerant having combustibility of a slight combustible level or higher (for example, 2L or higher in ASHRAE34 classification) is referred to as “combustible refrigerant”.
If combustible refrigerant is leaked into the room, the refrigerant concentration in the room may increase to form a combustible concentration region.
Patent Literature 1 describes an air-conditioning apparatus using combustible refrigerant, in which a gas sensor for detecting combustible refrigerant gas is provided on the outer surface of a floor type indoor unit, and the gas sensor is provided on the lower part of the indoor unit. When a sensor detection voltage by the gas sensor is a reference value or higher, the controller of the air-conditioning apparatus determines that combustible refrigerant is leaked, and gives warning immediately by an alarm. Thereby, a user is able to know leakage of combustible refrigerant, so that the user is able to take measures such as ventilating the room or calling a serviceman for repair. Further, when the controller determines that combustible refrigerant is leaked, the controller immediately performs control to stop operation of the refrigerant circuit. Thereby, even if the air-conditioning apparatus is in operation, the refrigerant circuit can be blocked immediately by the valve provided on the refrigerant circuit, whereby it is possible to suppress a large amount of leakage of the combustible refrigerant.
Meanwhile, Patent Literature 2 describes an air-conditioning apparatus including an outdoor unit, a heat medium relay unit, and an indoor unit. In the air-conditioning apparatus, the heat medium relay unit is provided in a space different from the inside of a room although it is in the building, such as a space above the ceiling. The heat medium relay unit is equipped with a relay unit fan for ventilation inside the casing. Further, the casing of the heat medium relay unit has an opening port formed at a position where the air of the relay unit fan passes through. The relay unit fan is always made driven with ventilation air amount or more (including the time when operation of the air-conditioning apparatus is stopped), for example, to suppress the refrigerant concentration inside the casing of the heat medium relay unit to be less than a lower limit combustion concentration (hereinafter referred to as “LFL”).